Water out alarm

ABSTRACT

The present invention provides for an improved method of determining a water out condition in a humidified gases supply apparatus. The method includes a two step process including a primary determination of a water out condition and a secondary determination of a water out condition. This primary determination is made during observation of the normal operation of the apparatus. During the secondary determination the method takes temporary control over the humidifying part of the apparatus. The secondary determination confirms or contradicts the primary determination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/174,485, filed Jun. 6, 2016, which is a continuation of U.S. patentapplication Ser. No. 13/643,901, filed Jan. 17, 2013, now U.S. Pat. No.9,393,379, issued Jul. 19, 2016, which is a national phase ofInternational Application No. PCT/NZ2011/000058, filed Apr. 26, 2011,which claims priority from U.S. Provisional Application No. 61/328,548,filed Apr. 27, 2010.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to humidification systems for gases to besupplied to a patient.

DESCRIPTION OF THE RELATED ART

For a range of applications, it is now known to be beneficial tohumidify gases being supplied to a patient. These applications includewhere the gases are for breathing by the patient and where the gas isbeing supplied during surgery to the patient. In the case of breathinggases, the humidity increases patient comfort and the humidified gasesare less prone to drying out the tissues of the patient airway. In thecase of surgical gases, the humidified gases reduce the drying out ofexposed tissue and improve post operative outcomes.

In a gases humidification system incorporating a humidification chamberfor humidifying gases for supply to the patient, it is important that acertain minimum level of water is maintained in order for the humidifierto have the ability to supply water vapour to the gases flow.Accordingly, the healthcare professional administering the humidifiedgases to the patient, or the patient themselves, in the case ofhome-based administration, should occasionally check the water level andadd more water when required. This task is sometimes overlooked.

U.S. Pat. No. 6,802,314 describes a respiratory humidification systemwhich automatically determines when the water level drops to aninsufficient level and raises an alarm. The system calculates,continuously, a measure of thermal conductivity as the power inputdivided by the difference between the heater plate temperature and thetemperature of the gases exiting the humidifier chamber. The controllercompares the calculated thermal conductivity with a predeterminedthreshold value suitable for the flow rate of gases in the system. Ifthe thermal conductivity is less than the threshold value, thecontroller activates an alarm.

This respiratory humidification system is intended for an environmenthaving fairly consistent ambient conditions. For example, hospitalwards, where these devices are most frequently used, tend to beair-conditioned and maintained at a comfortable temperature andbackground humidity.

Another example of an alarm system for alerting a user to replenish ahumidifying chamber is described in US patent application 2009/0184832.This patent application describes a humidifier for use in surgicalinsufflation. According to one embodiment, a controller measures thetotal power input to the humidifier heater over time. According to theapplicant, this represents the evaporation of a proportional amount ofwater from the reservoir. Once the total work (accumulated power over aperiod of time beginning when the chamber was filled) reaches apredetermined level, the controller indicates to the user that thehumidifier chamber may need refilling. In a further aspect described,the threshold level can be selected according to the temperature of thegases entering the system. According to another aspect, the systemdetermines that hydration fluid needs to be re-charged by determiningwhether the instantaneous power drops below a threshold, which could bedifferent for different flow rates or ranges of flow rates. According toanother variation, the controller monitors the temperature signal of theconditioned gas approximate or inside the heater hydrator and activatesan alarm when the temperature signal of the insufflation gas begins tovary significantly.

A still further system is described in US application 61/289,610, filed23 Dec. 2009 and assigned to Fisher & Paykel Healthcare Limited. Thisalso describes a humidification system for gases used in insufflation oropen-wound surgery. According to this application, a condition of low orno water in the humidifier chamber is detected by monitoring thetemperature of gases exiting the chamber while monitoring the powersupply to the heater base. If the temperature of the gases exiting thechamber drops at the same time as the power supplied to the heater isconstant or increasing, the controller determines this as a water-outcondition and alerts a user.

The surgical systems also operate within well-defined ambientconditions. For example, they are typically used in operating theatresmaintained at a regulated cool temperature and in an air-conditionedenvironment with relatively constant ambient humidity.

The inventors consider that each of the aforementioned systems issusceptible to false alarms. They are susceptible to determining thatthere is an absence of water in the chamber, and alerting the healthcareprofessional to this condition, in situations where the chamber is notactually empty. While an accidental false alarm is not critical in asituation where the system is being used by trained healthcareprofessionals, they are unhelpful in situations where the humidifiedgases delivery systems are being used outside the controlled hospitalenvironment. Systems used outside the controlled hospital environmentmay also be more susceptible to false alarms due to the wider range ofambient conditions in which they operate with ambient temperaturesranging, for example, from 12° C. to 32° C., and ambient humidity alsowidely varied. The varied ambient conditions are typical if the deviceis used in the home environment, such as devices used in CPAP therapy oroxygen therapy.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a gaseshumidification system which at least goes some way towards overcomingthe above disadvantages, which will at least provide the industry with auseful choice.

In one aspect, the present invention consists in an apparatus forpreparing a flow of gases including a heater for heating a reservoir, atemperature sensor in thermal communication with the heater to read thetemperature of the heater, a controller adapted to apply power from apower supply to the heater, the controller including a control methodcomprising monitor system conditions during use of the apparatus,determine a possible absence of water in the reservoir from themonitored system conditions, following a determination of a possibleabsence of water, apply power from the power supply to the heater, theamount of power being an increase in the supplied power, a maximum powerfrom the power supply to the heater, a predetermined amount of powerfrom the power supply to the heater; or an amount of power to the heaterthat would elevate the heater temperature in the absence of water, to atemperature that exceeds any temperature that the heater temperaturewould reach with water present in the chamber, monitor the output of thetemperature sensor, and reduce the power supplied from the power supplyto the heater, removing power from the heater completely if themonitored temperature sensor output indicated an absence of water in thereservoir.

According to a further aspect, the step of monitoring the output of thetemperature sensor includes determining a rise in heater platetemperature.

According to a further aspect, the step of monitoring the output of thetemperature sensor includes comparing the heater temperature against athreshold temperature and determining a water out condition where themonitored temperature exceeds the predetermined threshold temperature.

According to a further aspect, the controller determines the water outcondition when the monitored temperature exceeds the predeterminedthreshold temperature by a predetermined period of time.

According to a further aspect, the step of applying power to the heatercomprises applying a maximum available power to the heater.

According to a further aspect, the step of applying power to the heatercomprises applying a predetermined amount of power to the heater.

According to a further aspect, the step of applying power to the heatercomprises applying an amount of power to the heater that would elevatethe heater temperature in the absence of water to a temperature thatexceeds any temperature that the heater temperature would reach withwater present in the chamber.

According to a further aspect, the heater comprises a heater plate andthe apparatus includes a clamp for holding a removable chamber againstthe heater plate.

According to a further aspect, the temperature sensor is attached to theheater plate.

According to a further aspect, the apparatus for preparing a flow ofgases includes a blower for generating a flow of gases and a humidifierincorporating the heater and a reservoir of water, an outlet of theblower leading to an inlet of the humidifier.

According to a further aspect, the controller monitors system conditionsduring use of the apparatus including monitoring the temperature ofgases leaving the reservoir and monitoring power applied to the heaterand determines a possible water out condition based on a function of thetemperature of gases leaving the humidifier and the power applied to theheater.

According to a further aspect, the function comprises a ratio of thegases temperature to the heater plate power.

According to a further aspect, the controller determines a possibleabsence of water in the reservoir by comparing the result of thefunction against a predetermined threshold.

In a further aspect, the present invention consists in an apparatus forpreparing a flow of gases including a humidifier including a reservoirand a heater for heating the reservoir, a temperature sensor in thermalcommunication with the heater to read the temperature of the heater, acontroller adapted to apply power from a power supply to the heater, thecontroller including a control method comprising monitor systemconditions during use of the apparatus, determine a possible absence ofwater in the reservoir from the monitored system conditions, following adetermination of a possible absence of water, applying a maximumavailable power from the power supply to the heater, monitor the outputof the temperature sensor, determining a water out condition if theoutput of the temperature sensor exceeds a predetermined threshold for apredetermined period of time, subsequently reducing the power suppliedfrom the power supply to the heater, and providing an output indicatingthe water out condition

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

The term “comprising” is used in the specification and claims, means“consisting at least in part of”. When interpreting a statement in thisspecification and claims that includes “comprising”, features other thanthat or those prefaced by the term may also be present. Related termssuch as “comprise” and “comprises” are to be interpreted in the samemanner.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described withreference to the accompanying drawings.

FIG. 1 is a flow diagram illustrating the overall process fordetermining an absence of water in a humidifier chamber in accordancewith the present invention.

FIG. 2a shows a schematic view of a user receiving humidified air, withthe user wearing a nasal mask and receiving air from a modularblower/humidifier breathing assistance system.

FIG. 2b shows a schematic view of a user receiving humidified air, wherethe user is wearing a nasal cannula and receiving air from a modularblower/humidifier breathing assistance system.

FIG. 3 shows a schematic view of a user receiving humidified air, wherethe user is wearing a nasal mask and receiving air from an integratedblower/humidifier breathing assistance system.

FIG. 4 shows a schematic view of a user receiving humidified air, wherethe user is wearing a nasal cannula, the breathing assistance systemreceiving gases from a central source via a wall inlet and providingthese to a control unit, which provides the gases to a humidifierchamber in line with and downstream of the control unit.

FIG. 5 is a plot against time of a function of chamber outlettemperature and heater plate power and end of heater plate temperature,according to an experiment conducted using the present invention.

FIG. 6 is a flow diagram illustrating a preferred method to initiallydetermine a possible water out condition.

FIG. 7 shows a schematic representation of some of the connectionsbetween a controller suitable for use with the breathing assistancesystem of FIG. 2, 3 or 4, and other components of the preferred form ofbreathing assistance system as shown in FIG. 2, 3, or 4.

FIG. 8 is a flow diagram illustrating a confirmation method according toone embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides for an improved method of determining awater out condition in a humidified gases supply apparatus. This methodhas been found suitable for determining water out conditions in widelyvarying ambient conditions. As illustrated in FIG. 1, the methodincludes a two step process including a primary determination of a waterout condition at step 1010. This primary determination is made duringobservation (at step 1050) of the normal operation of the apparatus.Following a primary determination of a water out condition the methodtakes temporary control over the humidifying part of the apparatus atstep 1030 to make a secondary determination at step 1100 which willconfirm or contradict the primary determination.

For the primary determination, the controller of the apparatus maymonitor a range of characteristics of the system, as sensed by one ormore of the sensors included in the system. For example, the primarydetermination may be made by any one of a number of prior art methodsdescribed discussed in the background discussion. However, a preferredmethod for the primary determination monitors a function of the chamberoutlet temperature and the heater plate temperature and determines thepossibility of a water out condition when this function moves away froma base line value.

According to a preferred aspect of the present invention, the secondarydetermination, interrupts the normal control of the apparatus and takesover control of the humidifier heater power input. According to themethod for the secondary determination the heater power is increased toa maximum available value for a limited period of time. The secondarydetermination is then made based on the monitoring heater platetemperature (at step 1070). This determination may be on the basis ofthe heater plate temperature rising steeply during this time or stayingat an elevated level during this time, or reaching an elevated levelduring this time indicative of a water out condition.

The controller can then provide output of the water out condition (step1120) to activate a user alert. The controller also reduces or removespower to the heater plate (step 1110).

The water out determination method according to the present inventionhas been developed with particular application to humidified gasesdelivery apparatus used outside the hospital environment. However, themethod may also find beneficial in controllers for apparatus intendedfor the hospital environment.

The method can be used in the controller in a number of broad systemconfigurations. By way of example, three typical system configurationsare illustrated in FIGS. 2 to 4 and described below. These systemconfigurations are illustrative but not an exhaustive account of thesystem configurations in which this method may be used.

A schematic view of a user 2 receiving air from a modular assistedbreathing unit and humidifier system 1 according to a first examplesystem configuration is shown in FIGS. 2a and 2b . The system 1 providesa pressurised stream of heated, humidified gases to the user 2 fortherapeutic purposes (e.g. to reduce the incidence of obstructive sleepapnea, to provide CPAP therapy, to provide humidification fortherapeutic purposes, or similar). The system 1 is described in detailbelow.

The assisted breathing unit or blower unit 3 has an internal compressorunit, flow generator or fan unit 13—generally this could be referred toas a flow control mechanism. Air from atmosphere enters the housing ofthe blower unit 3 via an atmospheric inlet 40, and is drawn through thefan unit 13. The output of the fan unit 13 is adjustable—the fan speedis variable. The pressurised gases stream exits the fan unit 13 and theblower unit 3 and travels via a connection conduit 4 to a humidifierchamber 5, entering the humidifier chamber 5 via an entry port or inletport 23.

The humidifier chamber 5 in use contains a volume of water 20. In thepreferred embodiment, in use the humidifier chamber 5 is located on topof a humidifier base unit 21 which has a heater plate 12. The heaterplate 12 is powered to heat the base of the chamber 5 and thus heat thecontents of the chamber 5. As the water in the chamber 5 is heated itevaporates, and the gases within the humidifier chamber 5 (above thesurface of the water 20) become heated and humidified. The gases streamentering the humidifier chamber 5 via inlet port 23 passes over theheated water (or through these heated, humidified gases—applicable forlarge chamber and flow rates) and becomes heated and humidified as itdoes so. The gases stream then exits the humidifier chamber 5 via anexit port or outlet port 9 and enters a delivery conduit 6.

When a ‘humidifier unit’ is referred to in this specification withreference to the invention, this should be taken to mean at least thechamber 5, and if appropriate, the base unit 21 and heater plate 12.

The heated, humidified gases pass along the length of the deliveryconduit 6 and are provided to the patient or user 2 via a user interface7. The conduit 6 may be heated via a heater wire (not shown) or similarto help prevent rain-out.

The user interface 7 shown in FIG. 2a is a nasal mask which surroundsand covers the nose of the user 2. However, it should be noted that anasal cannula (as shown in FIG. 2b ), full face mask, tracheostomyfitting, or any other suitable user interface could be substituted forthe nasal mask shown. A central controller or control system 8 islocated in either the blower casing (controller 8 a) or the humidifierbase unit (controller 8 b). In modular systems of this type, it ispreferred that a separate blower controller 8 a and humidifiercontroller 8 b are used, and it is most preferred that the controllers 8a, 8 b are connected (e.g. by cables or similar) so they can communicatewith one another in use.

The control system 8 receives user input signals via user controls 11located on either the humidifier base unit 21, or on the blower unit 3,or both. In the preferred embodiments the controller 8 also receivesinput from sensors located at various points throughout the system 1.

FIG. 7 shows a schematic representation of some of the inputs andoutputs to and from the controller 8. It should be noted that not allthe possible connections and inputs and outputs are shown—FIG. 7 isrepresentative of some of the connections and is a representativeexample.

The sensors and their locations will be described in more detail below.In response to the user input from controls 11, and the signals receivedfrom the sensors, the control system 8 determines a control output whichin the preferred embodiment sends signals to adjust the power to thehumidifier chamber heater plate 12 and the speed of the fan 13. Theprogramming which determines how the controller determines the controloutput will be described in more detail below.

A schematic view of the user 2 receiving air from an integratedblower/humidifier system 100 according to a second form of the inventionis shown in FIG. 3. The system operates in a very similar manner to themodular system 1 shown in FIG. 2 and described above, except that thehumidifier chamber 105 has been integrated with the blower unit 103 toform an integrated unit 110. A pressurised gases stream is provided byfan unit 113 located inside the casing of the integrated unit 110. Thewater 120 in the humidifier chamber 105 is heated by heater plate 112(which is an integral part of the structure of the blower unit 103 inthis embodiment). Air enters the humidifier chamber 105 via an entryport 123, and exits the humidifier chamber 105 via exit port 109. Thegases stream is provided to the user 2 via a delivery conduit 106 and aninterface 107. The controller 108 is contained within the outer shell ofthe integrated unit 100. User controls 111 are located on the outersurface of the unit 100.

A schematic view of the user 2 receiving air from a further form ofbreathing assistance system 200 is shown in FIG. 4. The system 200 canbe generally characterised as a remote source system, and receives airfrom a remote source via a wall inlet 1000.

The wall inlet 1000 is connected via an inlet conduit 201 to a controlunit 202, which receives the gases from the inlet 1000. The control unit202 has sensors 250, 260, 280, 290 which measure the humidity,temperature and pressure and flow respectively of the incoming gasesstream.

The gases flow is then provided to a humidifier chamber 205, with thegases stream heated and humidified and provided to a user in a similarmanner to that outlined above. It should be noted that when ‘humidifierunit’ is referred to for a remote source system such as the system 200,this should be taken to mean as incorporating the control unit 202—thegases from the remote source can either be connected directly to aninlet, or via the control unit 202 (in order to reduce pressure orsimilar), but the control unit and the humidifier chamber should beinterpreted as belonging to an overall ‘humidifier unit’.

If required, the system 200 can provide O₂ or an O₂ fraction to theuser, by having the central source as an O₂ source, or by blendingatmospheric air with incoming O₂ from the central source via a venturi90 or similar located in the control unit 202. It is preferred that thecontrol unit 202 also has a valve or a similar mechanism to act as aflow control mechanism to adjust the flow rate of gases through thesystem 200.

Sensors

The modular and integrated systems 1, 100 and 200 shown in FIGS. 2, 3and 4 have sensors located at points throughout the system. These willbe described below in relation to the breathing assistance system 1.

The preferred form of modular system 1 as shown in FIG. 2 has at leastthe following sensors in the following preferred locations:

1) An ambient temperature sensor 60 located within, near, or on theblower casing, configured or adapted to measure the temperature of theincoming air from atmosphere. It is most preferred that temperaturesensor 60 is located in the gases stream after (downstream of) the fanunit 13, and as close to the inlet or entry to the humidifier chamber aspossible.

2) A humidifier unit exit port temperature sensor 63 located either atthe chamber exit port 9, or located at the apparatus end (opposite tothe patient end) of the delivery conduit 6. Outlet temperature sensor 63is configured or adapted to measure the temperature of the gases streamas it exits chamber 5 (in either configuration the exit port temperaturesensor 63 can be considered to be proximal to the chamber exit port 9).

Similarly, sensors are arranged in substantially the same locations inthe integrated system 100 shown in FIG. 3 and the system 200 of FIG. 4.For example, for the integrated system of FIG. 3, an ambient temperaturesensor 160 is located within the blower casing in the gases stream, justbefore (upstream of) the humidifier chamber entry port 123. A chamberexit port temperature sensor 163 is located either at the chamber exitport 109 and is configured to measure the temperature of the gasesstream as it exits chamber 105 (in either configuration the exit porttemperature sensor 163 can be considered to be proximal to the chamberexit port 109). Alternatively, this sensor can be located at theapparatus end (opposite to the patient end) of the delivery conduit 106,for either embodiment. A similar numbering system is used for thebreathing assistance system shown in FIG. 4—ambient temperature sensor260, fan unit 213, chamber exit port temperature sensor 263 located atthe chamber exit port 209, etc.

It is also preferred that the breathing assistance system 1 (and 100,200) has a heater plate temperature sensor 62 located adjacent to theheater plate 12, configured to measure the temperature of the heaterplate. The breathing assistance system(s) having a heater platetemperature sensor is preferred as it gives an immediate indication ofthe state of the heater plate. This sensor should be in the heat pathbetween the source of the heat and the reservoir. So, for example, asensor on a conductive plate that contacts the water chamber on one sideand has a heater on the other side is preferred.

It is also most preferred that the systems have a flow probe—flow probe61 in system 1—located upstream of the fan unit 13 and configured tomeasure the gases flow. The preferred location for the flow probe isupstream of the fan unit, although the flow probe can be locateddownstream of the fan, or anywhere else appropriate. Again, it ispreferred that a flow probe forms part of the system, but it is notabsolutely necessary for a flow probe to be part of the system.

The layout and operation of the breathing assistance system 1 will nowbe described below in detail. The operation and layout of the systems100 and 200 is substantially the same, and will not be described indetail except where necessary.

For the breathing assistance system 1, the readings from all of thesensors are fed back to the control system 8. The control system 8 alsoreceives input from the user controls 11.

Further Alternative Sensor Layouts

In a variant of the apparatus and method outlined above, the system(system 1 or system 100 or system 200) also has additional sensors asoutlined below.

1) A patient end temperature sensor 15 (or 115 or 215) is located at thepatient end of the delivery conduit 6 (or alternatively in or on theinterface 7). That is, at or close to the patient or point of delivery.When read in this specification, ‘patient end’ or ‘user end’ should betaken to mean either close to the user end of the delivery conduit (e.g.delivery conduit 6), or in or on the patient interface 7. This appliesunless a specific location is otherwise stated. In either configuration,patient end temperature sensor 15 can be considered to be at or close tothe user or patient 2.

Preferred Method for Primary Water Out Determination

With reference to FIG. 6, in the most preferred embodiment of thepresent invention the controller evaluates a function of the chamberoutlet temperature and the heater plate power on a continuing basisduring normal use of the apparatus. The controller determines thepotential for a water out condition when the result of this functionvaries from a base line level. The preferred function is a ratio of thechamber outlet temperature from sensor 63 (or 163 or 263) and the heaterplate power, as represented by the heater plate duty cycle. This ratiocan be represented as:

$\Theta = \frac{CBO\_ Temp}{{HP\_ duty}{\_ control}({average})}$

-   -   where    -   Θ=monitoring function,

CBO_Temp=Chamber outlet temperature from sensor 63,

HP_duty_control(average)=Heater plate duty control value, recorded eachsecond and averaged over 128 data points.

In tests conducted by the inventors, this ratio has been found to remainsubstantially constant in steady state conditions where water remains inthe humidification chamber, but to increase when the water runs out.This was the case across a range of ambient temperatures, includingambient temperatures from 8° C. to 32° C.

In the worst case, this ratio increased by a minimum of 10% from apre-existing base line value after the chamber ran dry.

A baseline value, θ₀, may be calculated in many ways. For example, θ₀could be a moving window average of θ, or a function of a moving windowaverage of CBO_Temp and a moving window average of the heater plate dutycycle. Alternatively, the value θ₀ could be periodically updated withthe present value θ, the period being, for example, 10 minutes or more.

An example of this monitoring function is illustrated in FIG. 5. FIG. 5includes a plot of this preferred function against time for a Fisher &Paykel Healthcare Ltd AIRVO humidified gases delivery apparatusoperating at 15 litres per minute with a Fisher & Paykel MR290humidification chamber, and an ambient temperature of 32° C. The plot isof a normalised ratio against time and of the heater plate temperatureagainst time. The normalised ratio is seen to remain steady untilapproximately t=1,300 s and then begin increasing, eventually reaching avalue of approximately 1.8.

Similar testing was conducted for two different humidifier chambers usedwith the Fisher & Paykel Ltd AIRVO humidified gases delivery apparatusacross three ambient temperatures and several flow rates. The testedconditions and flow rates represent the extreme operating conditions ofthe apparatus.

The normalised maximum of the evaluated ratio as the apparatus enteredthe water out condition is indicated in Table 1:

TABLE 1 Normalised maximum T_(CBO)/power ratio. HP Temperature/powerRatioθ/θ₀ 22° C. 32° C. 8° C. 15 L 45 L 15 L 45 L 15 L 45 L Chambermin⁻¹ min⁻¹ min⁻¹ min⁻¹ min⁻¹ min⁻¹ HC360 2.6 2.5 1.6 3.0 1.1 2.9 MR2901.2 5.2 1.8 3.1 1.4 2.7

Within this range of conditions two typical system responses to a waterout conditions were observed.

In a first type of response, the chamber outlet temperature decreasedwhile the heater plate temperature increased to a maximum value. Thiswas followed by the chamber outlet temperature increasing once more.

In the second type of response, the chamber outlet temperature increaseddirectly as the water out condition occurred, but the heater plate dutycontrol decreased while maintaining a set chamber outlet temperature. Inboth typical responses, the ratio of chamber outlet temperature toheater plate power increased compared to the value at the start of thewater out condition. In the case of the first type of response, a longertime is required for the ratio to increase relative to the base linevalue than for the second type of response.

This primary water out detection method is illustrated in FIG. 6. Step610 represents ongoing control of the heater plate power according tothe system conditions. This control aims to keep the delivered gasestemperature and humidity at or close to preferred levels.

As an ongoing process while this control continues, the method monitorsfor a possible water out condition based on a loop of steps 620 to 650.

At step 620, the method reads the chamber outlet temperature and theheater plate duty cycle. At step 630, the method calculates the ratio θ.At step 640, the method calculates θ₀ as a moving window average of themost recent values of the function θ.

At step 650, the method compares the value calculated at step 630 with amultiple of the value calculated at step 640. Alternatively, the methodcompares the value calculated at step 630 with a value calculated atstep 640 plus an arbitrary offset, namely the equation in step 650 ofFIG. 6 can be θ>θ₀+α, where a is constant for all temperatures or flowrates. In the exemplary embodiment, this multiple is 1.1, which has beenfound a usable threshold for an initial determination of a water outcondition across a range of use conditions for the humidified gasesdelivery apparatus tested. If the present value sufficiently exceeds thelong term value, the method proceeds to step 660 and returns a possiblewater out condition. If the present value is less than the long termvalue or not significantly greater than the long term value, the methodreturns to step 620 and continues to monitor the gases outlettemperature and heater plate duty cycle under the normal use conditions.

The example describes an implementation using normalised values, andstep 650 uses a threshold ratio. Alternatively, this step could becalculated to directly use measured values and determine a possiblewater out condition based on a threshold difference.

Preferred Method for Secondary Water Out Determination

According to the present invention a secondary determination of thewater out condition is made in response to a primary water outdetermination. According to the preferred embodiment, the method takescontrol of the humidifier power input, adjusts the heater plate powerinput and observes the temperature response of the heater plate. Thissecondary determination is implemented to confirm the water outcondition if the primary monitoring function returns a possible waterout condition.

The primary monitoring function may falsely determine a water out stateunder certain conditions. These conditions include disturbances in themains power supply, the occurrence of a sudden heat impulse to thechamber temperature sensor due to blocking of the flow or vigorousshaking of the unit causing water from the chamber to enter the outlettube. These imposed increases in chamber temperature will likely lead toa decrease in heater plate duty control and hence a change to the resultof the evaluation of the ratio, which could be incorrectly interpretedby the controller as a water out state.

According to the present invention, after the controller determines aprimary water out condition, the control program enters a routine forperforming a secondary water out determination.

As illustrated in FIG. 8, according to the preferred routine, thecontroller increases the heater plate duty cycle to the maximumavailable duty cycle, applying full power to the heater plate at step802. The control program proceeds to monitor the heater platetemperature at steps 806. and 808 until the heater plate temperatureexceeds 125° C. The control program then proceeds to monitor the timefor which the heater plate temperature continues to exceed 125° C. atsteps 810 to 814. If the heater plate temperature continuously exceeds125° C. for 30 s the controller determines a water out condition andsets a water out flag at step 816. The controller also reduces orremoves power to the heater plate at step 818.

If the control routine does not determine a water out condition, then atstep 820 the control routine returns control of the humidifier to themain operating control method. This can occur either because the heaterplate temperature does not exceed 125° C. for 30 seconds within 120seconds of applying maximum heater power at step 802. Implementing this,the method starts a first timer at step 804 and checks this timer atstep 822 after determining that the temperature is not above 125° C. atstep 808. If the timer is still less than 90 seconds, the methodproceeds to loop back to step 806, to read the updated heater platetemperature. If the timer exceeds 90 seconds, the method proceeds fromstep 822 to step 820, returning to normal control.

For the other condition for returning to normal control, the methodincludes a check at step 812 whether the heater plate temperature isstill above 125° C. In each loop, the method reads the heater platetemperature at step 826, and checks this temperature against thethreshold at step 812. If the present heater plate temperature exceedsthe threshold, the loop continues, otherwise the method proceeds to step822 and checks whether 90 seconds has elapsed since step 802.

This check is illustrated in the plots of FIG. 5. The start of thesecondary determination method is evident at t=1850 s. The heater platetemperature has been approximately 53° C. and slowly dropping. At t=1850s the heater plate temperature sharply rises, rapidly reaching valuesabove 125° C. and remaining at these elevated levels until thecontroller removes power from the heater plate in response to theconfirmed water out condition.

Essentially this secondary confirmation applies a predetermined heaterpower and monitors the response of the heater plate temperature anddetermines whether this response is characteristic of the chamber beingin a water out condition.

In the preferred method described, the applied power is the maximumavailable power. In some systems the applied power could be a lesservalue if it has been determined that a lower power was sufficient toachieve a response that can differentiate between a water out conditionand a condition where water remains in the chamber across all of theexpected operating conditions of the apparatus.

According to the preferred method, the controller monitors for anincrease in the heater plate temperature to a value exceeding apredetermined limit and for the heater plate temperature to remain abovethis limit continuously for a predetermined period of time.

Alternatively the controller could monitor for a rate of increase in theheater plate temperature that might have been shown by experimentationto be characteristic of a water out condition, or for some othercharacteristic of the heater plate temperature that has been determinedexperimentally to be characteristic of a water out condition, comparedto a condition in which there is water in the chamber.

The predetermined temperature values and times described in thispreferred embodiment of the present invention have been determined for aparticular humidified gases delivery apparatus. For other humidifiedgases delivery apparatus, suitable heater plate temperature thresholdsand periods would need to be determined by experimentation.

In the case of the apparatus on which this testing and experimentationwas conducted, tests have shown that in a water out condition, maximumapplied heater power achieves a heater plate temperature in excess of125° C. for a consistent and continuous time. Where water remains in thehumidified chamber, the heater plate temperature, with full duty cycleand in the highest ambient condition (32° C.) the heater platetemperature did not exceed 123° C. This is the worst case condition forthe secondary determination, and the condition under which the secondaryexamination could be most likely to provide a false positive for a waterout condition. Accordingly, for this device, the described secondarydetermination has been shown to reliably distinguish an empty chamberfrom a chamber with water remaining.

The invention has been described with particular reference to humidifiedgases delivery apparatus having a heater plate that contacts a heatconductive base of a water containing reservoir. The system of thepresent invention may be applied to other humidifier configurations. Forexample, the heater of the humidifier may reside within the waterreservoir or be integrated to the base or wall of the water reservoir.In either case, the temperature sensor for the secondary determinationshould be influenced by both the contents of the reservoir and the heatsource of the heater.

In these cases, it would be expected that the threshold temperature forthe secondary determination would be lower than the thresholdtemperature determined experimentally for the Fisher & Paykel HealthcareLtd AIRVO appliance.

In an appliance with a different heater arrangement, the heatertemperature might be determined by a thermistor in contact with theheating element, or in contact with a heat conductive substrate incontact with the heating element. Typically there will be a maximumtemperature that the substrate achieves for a given heater plate power(for example, maximum power) when there is water in the chamber due tothe limiting temperature at this boundary defined by the boiling pointof water. The heat supplied to the heater plate under these conditionsis absorbed as latent heat of vaporisation, and does not yield a changein temperature. In the absence of water, the latent heat energyrequirement disappears, and the excess energy over that required tomaintain steady state temperatures gives rise to an increase in gastemperature rather than the production of water vapour.

The preferred embodiment of the present invention has been described asproviding maximum power to the heater for a sustained period. Theinvention could be implemented so that a first elevated power isprovided to the heater to bring the heater above the thresholdtemperature, with the power subsequently reduced to a level thatmaintains the temperature above the threshold temperature, but does notfurther elevate of the temperature of the heater plate. This could beachieved for example, by using a closed feedback loop based on heaterplate temperature and could be particularly suitable where the poweravailable to be applied to the heater plate is much larger than thepower required to maintain the temperature above the threshold withtemperature of the heater applied above the threshold level with onlygases in the chamber.

1.-15. (canceled)
 16. An integrated humidified gases supply apparatuscomprising: a casing; a flow generator including a fan configured toprovide a flow of gases; a humidifier including a humidifier chamber anda heater plate, the humidifier chamber configured to contain water; anda controller, wherein the humidifier, the flow generator, and thecontroller are integrated in the casing, and wherein the controller isconfigured to adjust a flow rate output of the flow generator bycontrolling a speed of the fan, to monitor apparatus conditions duringuse of the apparatus and to determine a possible absence of water in thehumidifier chamber.
 17. The integrated humidified gases supply apparatusof claim 16, wherein the controller is configured to determine apossible absence of water by: monitoring a temperature of the gasesleaving the humidifier chamber, monitoring a power applied to the heaterplate, and determining a possible absence of water based on a functionof the temperature of the gases leaving the humidifier chamber and thepower applied to the heater plate.
 18. The integrated humidified gasessupply apparatus of claim 16, wherein the controller is configured todetermine a possible absence of water by: making a primary determinationof an absence of water, and making a secondary determination of anabsence of water.
 19. The integrated humidified gases supply apparatusof claim 18, wherein, in making the secondary determination, thecontroller is configured to interrupt a normal control of the apparatusand to take over control of a heater plate power input.
 20. Theintegrated humidified gases supply apparatus of claim 16, wherein, uponthe determination of the absence of water, the controller is configuredto output the determination of the absence of water to activate a useralert, and/or to reduce or remove a power applied to the heater plate.21. The integrated humidified gases supply apparatus of claim 16,wherein the apparatus is configured to provide oxygen or an oxygenfraction to a user.
 22. The integrated humidified gases supply apparatusof claim 21, comprising a flow control mechanism including a valveconfigured to control a flow rate of the gases and/or to provide anoxygen fraction to the user.
 23. The integrated humidified gases supplyapparatus of claim 16, comprising an ambient temperature sensor within,near, or on the casing, and/or a heater plate temperature sensor. 24.The integrated humidified gases supply apparatus of claim 16, comprisinga humidifier exit port temperature sensor at or proximal to an exit ofthe humidifier chamber.
 25. The integrated humidified gases supplyapparatus of claim 16, comprising a flow probe configured to measure aflow rate of the gases.
 26. An apparatus for supplying gases to apatient comprising: a casing; a flow generator including a fanconfigured to provide a flow of gases; a humidifier with a heater plate,an unsealed nasal cannula for a patient, a controller, and a valveoperable as a flow control mechanism to control an O2 fraction providedto the user, wherein the humidifier and flow generator are integrated inthe casing, the controller is configured to adjust the output of theflow generator by controlling the fan speed, and the controller isconfigured to monitor system conditions during use of the apparatus anddetermine a possible absence of water in the humidifier chamber based onthe monitored system conditions.
 27. The apparatus of claim 26, whereinthe controller is configured to determine a possible absence of waterby: monitoring the temperature of gases leaving the humidifier chamber,monitoring power applied to the heater plate, and determining a possibleabsence of water based on a function of the temperature of gases leavingthe humidifier and the power applied to the heater plate
 28. Theapparatus of claim 26, wherein the controller is configured to determinea possible absence of water by: making a primary determination of anabsence of water, and making a secondary determination of an absence ofwater.
 29. The apparatus of claim 28, wherein in making the secondarydetermination the controller is configured to interrupt the normalcontrol of the apparatus and take over control of the humidified heaterplate power input.
 30. The apparatus of claim 26, wherein the controlleris configured to output the absence of water determination to activate auser alert.
 31. The apparatus of claim 26, further comprising a deliveryconduit.
 32. The apparatus of claim 31, wherein the delivery conduitcomprises a heater wire.
 33. The apparatus of claim 26, wherein thecontroller is configured to operate the apparatus to provide O2 or an O2fraction to the user.
 34. The apparatus of claim 26, further comprisingan ambient temperature sensor located within, near or on the casing. 35.The apparatus of claim 26, further comprising a humidifier exit portsensor at or proximal to an exit of the humidifier chamber.